Integrated circuit load board and method having on-board test circuit

ABSTRACT

An integrated circuit load board includes a substrate on which a plurality of integrated circuit sockets and an integrated test circuit are mounted. The integrated test circuit generates test signals that are applied to the integrated circuit sockets. The integrated test circuit also receives response signals from the integrated circuit sockets indicative of the manner in which integrated circuits in the sockets responded to the test signals. Several of the load boards may be placed on a test head that may be coupled to a host.

TECHNICAL FIELD

This invention relates to testing devices for integrated circuits suchas memory devices, and, more particularly, to a relatively inexpensivemethod and system for high speed testing of integrated circuits.

BACKGROUND OF THE INVENTION

During the fabrication of such integrated circuits as memory devices, itis conventional to test the integrated circuits at several stages duringthe fabrication process. For example, the integrated circuits arenormally connected to a tester with a probe card when the integratedcircuits are still in wafer form. In a final test occurring after theintegrated circuits have been diced from the wafer and packaged, theintegrated circuits are placed into sockets on a load board. The loadboard is then placed on a test head, typically by a robotic handler. Thetest head makes electrical contact with conductors on the load board,and these conductors are connected to the integrated circuits. The testhead is connected through a cable to a high-speed tester so that thetester can apply signals to and receive signals from the integratedcircuits.

While the above-described testing environment works well in manyapplications, it is not without its limitations and disadvantages. Forexample, it is very difficult to test various timing characteristics ofthe integrated circuits, particularly at the high operating speeds forwhich such integrated circuits are designed. This difficulty resultsprimarily from the propagation delays in transferring signals throughthe cable between the tester to the test head. The cables that aretypically used in such testing environments are often fairly long, thusmaking the propagation delays of signals coupled to and from theintegrated circuits correspondingly long and often difficult to predict.

Another problem with the above-described testing environment is that itmay not accurately simulate the conditions in which the integratedcircuits will be actually used. In actual use, integrated circuits, suchas dynamic random access memory (“DRAM”) devices, are typically mountedon a printed circuit board. Signals are applied to the integratedcircuits by other integrated circuits mounted on the board, and signalsgenerated by the integrated circuits are received by other integratedcircuits mounted on the board. Most of the signals are not coupled toand from the integrated circuits through long cables coupled to distantelectronic devices. Therefore, the testing environment is normally quitedifferent from the environment in which the integrated circuits willoperate in normal use.

While techniques have been developed to deal with these difficulties,the use of these techniques results in testers that are highly complexand often very expensive. A large number of testers are normallyrequired for a high capacity semiconductor fabrication plant, thusgreatly increasing the cost of the plant and the expense of testing theintegrated circuits.

There is therefore a need for a testing system and method that caninexpensively test integrated circuits at high speed, and do so in anenvironment that accurately simulates the conditions in which theintegrated circuits are likely to be used in normal operation.

SUMMARY OF THE INVENTION

A system and method of testing integrated circuits includes a load boardon which the integrated circuits are removably mounted. The load boardincludes an integrated test circuit that generates test signalsresponsive to a test command coupled to the load board. As a result, thetest signals applied to the integrated circuits are generated on theload board itself. The integrated test circuit also receives testresults data from the integrated circuits on the load board. The testresults data may be indicative of the manner in which the integratedcircuits responded to the test signals. Information corresponding to thetest results data is then coupled from the load board. A plurality ofthe load boards may be removably placed on a test head. The test headapplies test commands to the load boards to cause the load boards toapply the test signals to the integrated circuits. The test head mayalso receive from the load boards the information corresponding to thetest results data. The test head may be coupled to a host to which itcan supply the information corresponding to the test results data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a test system 10 according to one exampleof the invention.

FIGS. 2A and 2B are top and bottom plan views, respectively, of a loadboard used in the test system of FIG. 1.

FIG. 3 is a block diagram of the test system of FIG. 1.

FIG. 4 is a more detailed block diagram of a test head used in the testsystem of FIG. 1.

FIG. 5 is a more detailed block diagram of a integrated test circuitused in the load board of FIGS. 2A and 2B.

FIG. 6 is an isometric view of a integrated circuit wafer probe cardcontaining an integrated test circuit according to one embodiment of theinvention.

DETAILED DESCRIPTION

A test system 10 according to one example of the invention isillustrated in FIG. 1. The test system 10 includes a test head 12 onwhich several load boards 14 a-d are placed. Each of the load boards 14a-d have a printed circuit substrate 16 on which several integratedcircuit sockets 18 a-h are mounted. Each of the integrated circuitsockets 18 a-h receives a respective integrated circuit (not shown inFIG. 1). In the example shown in FIG. 1, the sockets 18 a-h are adaptedto receive integrated circuit memory devices, such as dynamic randomaccess memory (“DRAM”) devices. Prior to final testing of the integratedcircuits, an integrated circuit handler (not shown) loads an integratedcircuit into each of the sockets 18 a-h, and then places each of theload boards 14 a-d on the test head 12.

As explained in greater detail below, each of the load boards 14 a-dincludes an integrated test circuit (not shown in FIG. 1) mounted on thesurface of the substrate 16 opposite the service on which the sockets 18a-h are mounted. The sockets 18 a-h are shown mounted on the uppersurface of the substrate 16 in greater detail in FIG. 2A. The lowersurface of the substrate 16 is shown in FIG. 2B. The integrated testcircuit 20 is mounted at the center on the lower surface of thesubstrate 16, and it is connected to the sockets 18 a-h throughconventional printed circuit traces (not shown in FIG. 2B). Also mountedon the lower surface of the substrate 16 are a conventional JTAGconnector 22 and a power supply connector 24. As explained in greaterdetail below, a test file containing test instructions, such as thecharacteristics of memory commands used by memory devices in the sockets18 a-h or the address range to be tested, are applied to the integratedtest circuit 20 through the JTAG connector 22. The test file is storedin random access memory (“RAM”) internal to the integrated test circuit20. Results data are subsequently received from the integrated testcircuit 20 through the JTAG connector 22. Power is applied to theintegrated test circuit 20 as well as to integrated circuits mounted inthe sockets 18 a-h through the power supply connector 24. The integratedtest circuit 20 includes a command generator (not shown) that generatesmemory command signals, and an address generator (not shown) thatgenerates memory address signals. The integrated test circuit 20 alsocontains a write data generator (not shown) including a patterngenerator that generates a pattern of write data signals.

In operation, the integrated test circuit 20 write a pattern of signalsto the integrated circuits mounted in the sockets 18 a-h by couplingappropriate command and address signals, as well as the pattern of writedata, to the integrated circuits mounted in the sockets 18 a-h. Inresponse to coupling appropriate command and address signals generatedby the integrated test circuit 20, the integrated test circuit 20receives read data from the integrated circuits. The integrated testcircuit 20 compares the read data to the write data to determine if theintegrated circuits in the sockets 18 a-h are functioning properly.Insofar as the integrated test circuit 20 is mounted on the samesubstrate 16 as the integrated circuits, the timing at which the signalsin the pattern are applied to the integrated circuits can be moreprecisely controlled. Furthermore, the relatively short length of theconductors extending from the integrated test circuit 20 to theintegrated circuits in the sockets 18 a-h allows the circuitry in theintegrated test circuit 20 to be less complex and therefore lessexpensive than with testers in which a pattern of test signals isapplied through a long cable.

With further reference to FIG. 1, the test head 12 is mounted on theupper ends of several power supplies 26 a-d, which generate DC supplyvoltages at respective magnitudes for use by the test head 12 and theload boards 14 a-d. As mentioned above, the DC supply voltages areapplied to the test head 12 through the power supply connector 24 (FIG.2B). The test head 12 also includes fluid couplings 29 a,b that receiveand discharge a cooling fluid, respectively, for maintaining thetemperature of the integrated test circuit 20 at a constant acceptablycool temperature.

The basic components of a test system 30 that includes the test head 12and load boards 14 a-d, as well as a host, is shown in FIG. 3. In theexample shown in FIG. 3, eight memory devices 34 a-h are mounted on eachof the load boards 14 a-d (only one load board 14 is shown in FIG. 3).The memory devices 34 a-h are coupled to the integrated test circuit 20through a common uni-directional address/control bus and a commonbi-directional data bus.

The test head 12 includes interface circuitry 50, including an embeddedCPU 52. The interface circuitry 50 is coupled to the load board 14through a conventional JTAG bus 54 and the JTAG connector 22 (FIG. 2B).The interface circuitry 50 supplies power to the load board 14 throughpower supply lines 56 and the power supply connector 24 (FIG. 2B).Finally, the interface circuitry 50 includes temperature control lines58 that provide signals indicative of the temperature of the load board14. The interface circuitry 50 includes a test program, which may bestored in a programmable read only memory (“PROM”) 60 included in theinterface circuitry 50. This test program performs tests on aprogrammable system clock, address/voltage multiplexers, and temperaturecontrol circuits 64 which are included in the interface circuitry alongwith the CPU 52

In operation, the interface circuitry 50 receives a test file from ahost 70, which may be a conventional personal computer. The host 70 iscoupled to the interface circuitry 50 through suitable means, such as aLocal Ethernet network 74. The host 70 may also be coupled to a largernetwork, such as a production management system, through a CorporateEthernet network 76. The interface circuitry 50 then passes the testfile onto the integrated test circuit 20, which then applies a patternof write data signals to the integrated circuits 34 a-h with preciselycontrolled timing. The integrated test circuit 20 then receives readdata signals from the integrated circuits 34 a-h and outputs test resultdata to the CPU 52. The CPU 52 outputs the test data to the host 70. Inthe event the integrated circuits 34 a-h are devices other than memorydevices, the integrated test circuit 20 generates signals other thanmemory command, address and data signals to write a pattern of data tothe memory devices with precisely controlled timing. After the patternof data have been written to the memory devices, the integrated testcircuit 20 generates command and address signals to read the contents ofthe memory devices, and it compares the read data with the write data todetermine if the memory devices properly stored the write data. Theintegrated test circuit 20 formulates the test results into high-leveltest data, which it passes to the test head 12 through the JTAG bus 54.The test head 12, in turn, collects the test data and sends it to thehost 70 through the Local Ethernet network 74.

The circuitry in the load board 14 and the test head 12 are shown ingreater detail in FIG. 4. FIG. 4 shows the integrated test circuit 20coupled to the memory devices 34 a-h. The integrated test circuit 20 hasa system management bus (“SMBus”) that is coupled to several componentsin the test head 12. More specifically, the SMBus is coupled through avoltage translation buffer 78 to an analog-to-digital converter 80, adigital-to-analog converter 82, a board identifier 86, which outputsdata indicative of the identity of the load board 14, and a CPU 52. Theintegrated test circuit 20 includes a Bandgap reference generator (notshown) that outputs a precisely controlled current. This current isconverted to a reference voltage V_(R) by passing the reference currentthrough a resistor 89 in the test head 12. The reference voltage V_(R)is applied to the analog-to-digital converter 80, which also receives amonitored voltage V_(M). The analog-to-digital converter 80 outputs adigital value indicative of either the reference voltage V_(R) or themonitored voltage V_(M), and this digital value is coupled to the CPU 52through the SMBus.

The test head 12 receives supply voltages from the power supplies 26 a-d(FIG. 1), and includes voltage regulators 90, 92, 94 that regulate thepower supply voltages to fixed magnitudes of 1.5V, 1.2V and 1.8V. Thetest head 12 also includes adjustable voltage regulators 100, 102 thatgenerate variable voltages. The magnitudes of the variable voltages arecontrolled by the magnitudes of voltage adjust signals generated by thedigital-to-analog converter 82 responsive to digital signals receivedfrom the CPU chip 52 through the SMBus.

The integrated test circuit 20 also receives a clock signal from a clockgenerator 110 having a frequency that is determined by a digital signalfrom the integrated test circuit 20. The frequency of the clock signalcontrols the rate at which command, address and write data signals aretransferred to the memory devices 34 a-h and read data are transferredfrom the memory devices 34 a-h. The integrated test circuit 20 istherefore able to test the memory devices 34 a-h over a wide range ofoperating speeds.

As explained above, the integrated test circuit 20 outputs test resultsthrough the JTAG bus 54. The JTAG bus 54 is coupled to voltagetranslation buffers 120, which convert the signals from logic levelsused by the integrated test circuit 20 to logic levels used by the host70, which, as explained above with reference to FIG. 3, is coupled tothe test head 12.

Finally, the integrated test circuit 20 is coupled to an electricallyeraseable programmable read-only memory (“PROM”) 124, which containscalibration data that precisely characterizes the timing characteristicsof the integrated test circuit 20. As a result, differences in thetiming characteristics of the signals generated by the integrated testcircuit 20 can be compensated for so that the timing characteristics ofthe signals applied to integrated circuits being tested can be preciselycontrolled.

In operation, the CPU 52 applies test commands to the integrated testcircuit 20, as explained above. These test commands might be, forexample, simply a command to write specific data values to a specificaddress in the memory devices 34 a-h. The integrated test circuit 20responds to the test commands by generating command, address and writedata signals that cause the write data to be written to the memorydevice 34 a-h. At the same time, the analog-to-digital converter 80monitors the reference voltage from the resistor 89 and the adjustablevoltage regulators 100, 102. After comparing the monitored voltagesV_(M) from the regulators 100, 102 to the reference voltage V_(R), theCPU 52 adjusts the variable voltages to the desired values by outputtingcorresponding voltage control data to the digital-to-analog converter82. As a result, the integrated test circuit can test the memory devices34 a-h with different supply voltage values, at different operatingspeeds, and, as explained below, with different timings of the command,address and data signals.

The architecture of the integrated test circuit 20 is shown in greaterdetail in FIG. 5. The integrated test circuit 20 includes severalinterfaces for coupling signals to and from the DUT chip 20. A firstSNBus interface 130 is coupled to the analog-to-digital converter 80,the digital-to-analog converter 82, the board ID 86, and the CPU 52(FIG. 4) through a SMB0 bus 132. A second SMBus interface 136 is coupledto the PROM 124 (FIG. 4) through a SMB1 bus 138. A general-purposeinput/output (“GPIO”) interface 140 is likewise coupled to the clockgenerator 110 (FIG. 4) through a GPIO bus 142. The JTAG test commandsand result data are coupled through the JTAG bus 54 and a JTAG interface150. Finally, a memory interface 160 includes a first unidirectionaladdress/command interface 162 for coupling address and command signalsto four of the memory devices 34 a-d, a second unidirectionaladdress/command interface 164 for coupling address and command signalsto the remaining four memory devices 34 e-h, and a bidirectional datainterface 166 for coupling write data to and read data from the memorydevices 34 a-h.

The timing of signals coupled from the integrated test circuit 20 to thememory device 34 is derived from internal clock signals generated by aphase locked loop (“PLL”) 170. As is well-known in the art, the phaselocked loop 170 includes a voltage controlled oscillator having a phasethat is locked to the phase of a reference clock signal CLK_(R). Thereference clock signal is, in this case, the clock signal from the clockgenerator 110 (FIG. 4). The phase locked loop 170 includes aconventional phase interpolator so that the clock signals generated bythe phase locked loop 170 can have a variety of phases relative to thephase of the reference clock signal. The internal clock signals arecoupled to the JTAG interface 150, and from the JTAG interface 150 to analgorithmic pattern generator (“APG”) 174. The automatic patterngenerator 174 generates a pattern of write data that is stored in thememory devices 34 a-h. This pattern of data is coupled to a clock domaincrossing circuit 178, which allows the data to pass from the clockdomain of the reference clock signal from the clock generator 110 to theclock domain of the memory device 34. By using a variety of phases ofthe internal clock signal from the phase locked loop 170, the integratedtest circuit 20 is able to vary the timing of command, address and writedata signals applied to the memory devices 34 a-h to test the timingmargins of the memory device 34 a-h.

Finally, the integrated test circuit 20 includes a configuration switch180 and configuration logic 184 that function together to configure theintegrated test circuit 20 based on the characteristics of the memorydevices 34 a-h.

Although the integrated test circuit 20 may be mounted on a load board14 as described above with reference to FIGS. 1-5, it may also be usedin other contexts, such as on an integrated circuit probe card 200 asshown in FIG. 6. The probe card 200 includes a substrate 204 on whichthe integrated test circuit 20 is mounted. The test circuit 20 iscoupled to a host 210 through a suitable communications link 214, suchas a JTAG interface. The integrated circuit test circuit 20 is alsocoupled to conductive integrated circuits probes 220. The ends of theprobes 220 make contact with pads formed on integrated circuits 224fabricated on a wafer 228. The integrated circuit test circuit 20operates in substantially the same manner as described above to test theintegrated circuits 224 and report the results of the tests to the host210.

Although the present invention has been described with reference to thedisclosed examples, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Such modifications are well within the skillof those ordinarily skilled in the art. Accordingly, the invention isnot limited except as by the appended claims.

1. An integrated circuit load board, comprising: a substrate; aplurality of integrated circuit sockets mounted on the substrate; anintegrated test circuit mounted on the substrate and coupled to theintegrated circuit sockets, the integrated test circuit being operableto apply test signals to the integrated circuit sockets and to receiveresponse signals from the integrated circuit sockets, the integratedtest circuit having an output port from which test results data derivedfrom the response signals are output; and a configuration switchoperable to provide configuration data that is coupled to the integratedtest circuit, the integrated test circuit using the configuration datato control the manner in which the test signals are applied to theintegrated circuit sockets.
 2. The integrated circuit load board ofclaim 1, further comprising a respective integrated circuit in each ofthe integrated circuit sockets in communication with the integrated testcircuit.
 3. The integrated circuit load board of claim 2 wherein each ofthe integrated circuits comprise a dynamic random access memory device.4. The integrated circuit load board of claim 1, further comprising acontrol register to store the test results data.
 5. The integratedcircuit load board of claim 1, wherein the integrated test circuitcomprise: a pattern generator operable to generate a pattern of testsignals; and an integrated circuit interface coupled to the patterngenerator and to the integrated circuit sockets, the pattern generatorand integrated circuit interface being operable to apply the pattern oftest signals to the integrated circuit sockets.
 6. An integrated circuitload board comprising: a substrate; a plurality of integrated circuitsockets mounted on the substrate; an integrated test circuit mounted onthe substrate and coupled to the integrated circuit sockets, theintegrated test circuit being operable to apply test signals to theintegrated circuit sockets and to receive response signals from theintegrated circuit sockets, the integrated test circuit having an outputport from which test results data derived from the response signals areoutput, the integrated test circuit comprising a pattern generatoroperable to generate a pattern of test signals and an integrated circuitinterface coupled to the pattern generator and to the integrated circuitsockets, the pattern generator and integrated circuit interface beingoperable to apply the pattern of test signals to the integrated circuitsockets; and wherein the integrated circuit interface is responsive toan internal clock signal to apply the pattern of test signals to theintegrated circuit sockets, and wherein the integrated circuit loadboard further comprises a clock control circuit generating the internalclock signal responsive to a reference clock signal, the clock controlcircuit being operable to adjust the phase of the internal clock signalrelative to the phase of the reference clock signal to vary the timesthat the pattern of test signals are applied to the integrated circuitsockets.
 7. An integrated circuit load board comprising: a substrate; aplurality of integrated circuit sockets mounted on the substrate; anintegrated test circuit mounted on the substrate and coupled to theintegrated circuit sockets, the integrated test circuit being operableto apply test signals to the integrated circuit sockets and to receiveresponse signals from the integrated circuit sockets, the integratedtest circuit having an output port from which test results data derivedfrom the response signals are output, the integrated test circuitcomprising a pattern generator operable to generate a pattern of testsignals and an integrated circuit interface coupled to the patterngenerator and to the integrated circuit sockets, the pattern generatorand integrated circuit interface being operable to apply the pattern oftest signals to the integrated circuit sockets; and a clock domaincrossing buffer coupling the pattern generator to the integrated circuitinterface, the clock domain crossing buffer being operable to transferthe pattern of test signals from the clock domain of the patterngenerator to the clock domain of the integrated circuit interface.
 8. Atest system for integrated circuits, the test system comprising: aplurality of load boards, each of the load boards comprising: asubstrate; a plurality of integrated circuit sockets mounted on thesubstrate; and an integrated test circuit mounted on the substrate andcoupled to the integrated circuit sockets, the integrated test circuitbeing operable to apply test signals to the integrated circuit socketsresponsive to a test command, the integrated test circuit further beingoperable to receive response signals from the integrated circuit socketsand to provide test results data derived from the response signals; atest head on which the load boards are removably mounted, the test headhaving a plurality of sets of load board terminals in electricalcommunication with the load boards and a set of host interfaceterminals, the test head being operable to apply the test commands tothe load boards and to receive the test results data from the loadboards; wherein the test head further comprises a voltage regulatorcoupled to the at least one power supply, the voltage regulator beingoperable to compare the voltage of the electrical power applied to thetest head from the at least one power supply to a reference voltage andto generate adjust signals to which the at least one power supply isresponsive to vary the voltage of the electrical power until the voltageis substantially equal to the reference voltage.
 9. The test system ofclaim 8, further comprising a host coupled to the test head, the hostbeing operable to apply high level commands to the test head and toreceive the test results data from the test head, the test head furtherbeing operable to generate the test commands responsive to the highlevel commands.
 10. The test system of claim 8, further comprising atleast one power supply providing electrical power to the test head andthe load boards.
 11. The test system of claim 8 wherein the voltageregulator comprises: an analog-to-digital converter receiving thereference voltage and the voltage of the electrical power and generatingrespective first and second digital values corresponding thereto; aprocessor coupled to receive the first and second digital values fromthe analog-to-digital converter, the processor generating a digitalcontrol word corresponding to the difference between the first digitalvalue and the second digital value; and a digital-to-analog convertercoupled to the processor, the digital-to-analog converter being operableto generate a voltage adjust signal having a magnitude corresponding tothe digital control word.
 12. The test head of claim 8, furthercomprising a cooling system coupled to the test head to remove heat fromthe test head.
 13. The test system of claim 8 wherein each of the loadboards further comprises a respective integrated circuit in each of theintegrated circuit sockets in communication with the integrated testcircuit on the load board.
 14. The test system of claim 13 wherein eachof the integrated circuits comprise a dynamic random access memorydevice.
 15. The test system of claim 8 wherein the respective integratedtest circuit in each of the load boards comprises: a pattern generatoroperable to generate a pattern of test signals; and an integratedcircuit interface coupled to the pattern generator and to the integratedcircuit sockets, the pattern generator and integrated circuit interfacebeing operable to apply the pattern of test signals to the integratedcircuit sockets.
 16. The test system of claim 15 wherein the integratedcircuit interface in each of the integrated test circuits is responsiveto an internal clock signal to apply the pattern of test signals to theintegrated circuit sockets, and wherein each of the integrated circuitload boards further comprises a clock control circuit generating theinternal clock signal responsive to a reference clock signal, the clockcontrol circuit being operable to adjust the phase of the internal clocksignal relative to the phase of the reference clock signal to vary thetimes that the pattern of test signals are applied to the integratedcircuit sockets.
 17. The test system of claim 15 wherein each of theintegrated test circuits further comprises a clock domain crossingbuffer coupling the respective pattern generator to the respectiveintegrated circuit interface, the clock domain crossing buffer beingoperable to transfer the pattern of test signals from the clock domainof the pattern generator to the clock domain of the integrated circuitinterface.
 18. The test system of claim 8 wherein the integrated testcircuit in each of the load boards further comprises a control registerto store the test results data.
 19. The test system of claim 8 whereinthe integrated test circuit in each of the load boards further comprisesa respective configuration switch operable to provide configuration datathat is coupled to the integrated test circuit, the integrated testcircuit using the configuration data to control the manner in which thetest signals are applied to the integrated circuit sockets.
 20. A methodof testing integrated circuits, comprising: loading a plurality of theintegrated circuits on a load board; generating a pattern of testsignals on the load board responsive to a test command coupled to theload board; coupling a clock signal to the integrated circuits; applyingthe pattern of test signals to the integrated circuits on the loadboard, the act of applying the pattern of test signals to the integratedcircuits comprising adjusting the timing at which the pattern of testsignals are applied to the integrated circuits relative to the timing ofthe clock signal; receiving test results data from the integratedcircuits on the load board, the test results data being indicative ofthe manner in which the integrated circuits responded to the testsignals; and coupling information corresponding to the test results datafrom the load board.
 21. The method of claim 20 wherein each of theintegrated circuits comprises an integrated memory device.
 22. Themethod of claim 21 wherein each of the memory devices comprises anintegrated dynamic random access memory device.
 23. The method of claim21 wherein the test signals comprise write data signals, and wherein thetest results data comprise data read from the memory devices.
 24. Themethod of claim 20 wherein the test signals comprise a pattern of testsignals, the pattern of test signals being applied to the integratedcircuits.
 25. The method of claim 20, further comprising storing thetest results data in the load board.
 26. The method of claim 20 whereinthe act of applying the test signals to the integrated circuits on theload board comprises varying an output impedance at which it applies thetest signals to the integrated circuit sockets.